It depends. I want the least airflow obstruction. If I have a narrow grill, then I'm going with a bar. I prefer to have two light heads lit up at all time. It's my OCD, but I can't tolerate the look of my vehicle being asymmetrical. If I had a large grill, say a Ford F-series, I would go with 4 larger light heads.
Example: I used to have a '96 Ford Bronco about 10 years ago. That thing had a giant grill. It had a pair of Tomar 3x7 linear strobes in the upper corners, and a pair of Code 3 3x7 halogens in the bottom corners. Today, I have a S-10 Blazer. It has a fairly low profile grill. I have no choice but to mount my sirens speakers behind the grill, has that's the only opening in the front. I plan on putting an E66 in the top portion.